Federal government regulations require close monitoring of automotive fuel system vapor pressure. Sensitive electronic components are used as part of an On-board Diagnostic System (OBD II) to detect hydrocarbon leaks in a fuel system. However, the fuel system environment is extremely harsh.
Several attempts have been made to adapt integrated electronic sensors, normally used in ambient and atmospheric applications, so that they can withstand the hostile environment of an automotive fuel system. For example, placing the fragile sensing cell of a pressure sensor in a thermoplastic housing.
U.S. Pat. No. 5,465,626 to Brown et al. provides a pressure sensor in which the pressure sensitive sensing cell is attached to a stress isolation platform using an adhesive that has a similar thermal coefficient of expansion and provides a hermetic seal between the isolation platform and the sensing cell. The stress isolation platform is attached to a plastic sensor housing with semi-rigid adhesive providing stress isolation and a hermetic seal between the plastic housing and the stress isolation platform. The hermetic seal between the housing and the stress isolation platform and the hermetic seal between the sensing cell and the stress isolation platform isolates the sensitive electronics of the sensor from the harsh environment it is exposed to. However, the adhesive used to hermetically seal the isolator is directly exposed to the harsh environment.
The stress isolation platform is made of glass. A special blend of glass is required in order to withstand the thermal requirements of the sensor. Special bulk processing of the glass is required, which limits the number of suppliers capable of processing the glass. In addition, close tolerances of the glass after firing are required which adds significantly to the cost of the glass. The strength of glass is not easily quantifiable, adding to the potential for cracked glass during the sensor assembly process and leading to high scrap rates.
In an attempt to avoid the problems associated with adhesive failure, U.S. Pat. No. 5,461,922 to Koen provides a pressure transducer that has a header housing made of a thermoplastic material. The housing has a resilient diaphragm made of a material that includes the same base material as the housing, but is flexible. The sensing cell is secured to a circuit board and the circuit board is secured to the header by and adhesive bond. The header and diaphragm provide a fully enclosed housing that is filled with a pressure transfer medium to couple the resilient diaphragm to the sensing cell, without exposing the sensing cell to any harsh chemicals. So while the housing of the pressure sensor protects the adhesive from exposure to harsh chemicals, the housing filled with transfer medium filled is extremely complicated and costly to manufacture. In addition, the reliability of the sensor is questionable due to the need for additional materials, such as the circuit board and the transfer medium, and the potential for the transfer medium to leak from the housing.